Advances in carbonate exploration location:London date:4-5 November 2010
A multidisciplinary approach in the study of karstified carbonate reservoirs is applied by Eni E&P on different reservoir analogue outcrops. The study of the relationships between the sedimentary framework, tectonic evolution and subaerial exposure characteristics in three-dimensional outcrops in Apulia is a key to interpret and forecast the porosity-permeability properties distribution in Southern Italy’s carbonate reservoirs.
Since the Upper Cretaceous the Apulia carbonate platform underwent several phases of exhumation and karstification with the creation of vugs and caverns in different shapes and sizes. Apart from the duration of exposure and intensity of sea-level fall, the distribution as well as the organisation of the karst cavities are influenced by several factors. The aim of this study is to define the dominant factors for the observed karst types.
Sedimentological (depositional environment, bedding texture) and diagenetic (small scale dissolution, stylolites, cementation, fracturing) aspects have been studied in the field, hand samples and thin sections. Mechanical stratigraphic analyses has been performed on different structural datasets (fracture orientation, length, opening) obtained by field measurements, high resolution photography and LIDAR acquisition. Karstification phenomena have been described based on field characteristics and controlling parameters were explored. Cement phases and (paleo-)speleothems were sampled and their stable isotope composition was determined. To refine the relative stratigraphic setting of the outcrops studied, chemostratigraphic and biostratigraphic analyses have been carried out.
For this study, 4 quarry outcrops are studied and sampled in the Murge area. In these quarries, the limestones of the Apulia platform cover a range from Aptian to Senonian in age. Two of the quarries are located close to the coast near the towns of Trani and Bisceglie and consist of Aptian inter- and subtidal mud- to wackestones. The Cavallerizza and Barile quarries are located 20 km inland and consist of Cenomanian rudist dominated subtidal mudstone. Towards the top of these mudstones a well-developed paleosol occurs overlain by Senonian peritidal deposits.
Stable C- and O-isotope measurements combined with biostratigraphical data allowed to position the different outcrops in a stratigraphical framework. Hardgrounds preserved their original signature better due to early marine cementation and are a tool for outcrop correlation. A paleosol separates the Cenomanian from the Senonian deposits. A distinct negative δ13C-excursion occurs at this level and relates to soil-forming diagenesis. The overlying Senonian deposits are 2 ‰ δ13C more depleted than the Cenomanian rudist banks. Due to the impermeable nature of the paleosol, the Cenomanian deposits were protected from meteoric fluids that could only penetrate where the paleosol was breached by fractures. Around karst pockets an influence zone is present that extends 1 m in the host rock. Within this zone δ13C-depletion of 1,4 ‰ is observed, which fades out exponentially with distance from the karst cavity. Due to lower water-rock interaction, calcite precipitation within large vugs and karst cavities has more depleted δ13C signatures than in smaller pores (e.g.: molds).
Porosity measurements revealed porosity is present on a wide spectrum. Porosity of the host rock is high (up to 25%) but is mainly limited to microporosity and molds. Karstification and solution enlarged fractures add ca. 5%. The latter pores make up a well connected pore network. Total porosity can thus attain 30%.
Based on fracture orientation data extracted from LIDAR scans different orientation clusters were observed between fractures that were karstified and fractures that were not karstified, which thus postdate karstification. The clusters of karstified fractures are related to the compression of the southern Apennines and were dissolution enlarged during the uplift caused by bulging of the Apulia platform (Pleistocene). This main karstification phase occurred prior to Late-Pleistocene deposition and before orthogonal fracture sets formed, which are not karstified.
Mechanical stratigraphy was used to calculate the fracturing within different sedimentary layers. Several mechanical units were defined and a characteristic relation was found between unit thicknesses and fracture intensity. Fracture intensity increases if layer thickness decreases. In some of the quarries sedimentary cycles were clearly present. These could then also be observed in fracture intensity logs. The degree of karstification within a mechanical unit is inversely proportional to the respective fracture intensity.
Several karstification types are recognized in the Murge area. Paleokarstification comprises small scale bioclasts dissolution and karstification of the loose sediments below hardgrounds. When the karst cavity exceeded the maximum span for the hardgrounds they collapsed and filled the void.
For the “Subrecent” karst that formed during the Pleistocene three concepts are proposed.
1) Karst cavities are formed below an impermeable paleosol along fractures that crosscut this paleosol. These fractures are the main infiltration paths for meteoric fluids to reach the rock below.
2) In layered rocks karstification is controlled by fracture intensity of the layers (mechanical stratigraphy). Karstification mainly occurs in layers where fracture intensity is low and fluid flow is focussed through only few fractures.
3) The normal faults in the area are preferential pathways for meteoric fluids. The footwall of normal faults is readily affected by karstification. Fluids that pass through normal faults can be injected into mechanical stratigraphic controlled karst features.
In this study, the combination of several techniques led to the conclusion that karstification in the Murge area is mostly controlled by fracturing. The fractures are the result of the tectonic regime implied by the southern Apennines orogeny, by forming compressional conjugate fracture sets, normal faults and dextral transpressional fractures. Mechanical stratigraphy is a tool to examine the fracture intensity within layers and mechanical units. The thickness of mechanical units is inverse proportionally related to the fracture intensity which then controls the amount of karstification within that unit.